Generally speaking, land vehicles require three basic components. These components comprise a power plant (such as an internal combustion engine), a power train and wheels. The internal combustion engine produces force by the conversion of the chemical energy in a liquid fuel into the mechanical energy of motion (kinetic energy). The function of the power train is to transmit this resultant force to the wheels to provide movement of the vehicle.
The power train's main component is typically referred to as the transmission. Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle. The vehicle's transmission is also capable of controlling the direction of rotation being applied to the wheels, so that the vehicle may be driven in both the forward and backward directions.
A conventional automatic transmission includes a hydrodynamic torque converter to controllably transfer engine torque from the engine crankshaft to a rotatable input member of the transmission through fluid-flow forces. The transmission also includes frictional units which couple the rotating input member to one or more members of a gearset. Other frictional units, typically referred to as brakes, hold members of the gearset stationary during the flow of power. These frictional units are usually brake clutch assemblies or band brakes. The drive clutch assemblies can couple the rotating input member of the transmission to the desired elements of the gearsets, while the brakes hold elements of these gearsets stationary. Some types of automatic transmissions include one or more planetary gearsets in order to provide various ratios of torque and to ensure that the available torque and the respective tractive power demand are matched to one another. Other types of automatic transmissions do not have planetary gearsets, utilizing instead a gearset resembling that of a manual transmission and having selectable gear ratios that are selected by electrically actuated pairs of gears in conjunction with a hydraulic clutch.
In contrast to the manual variety, automatic transmissions are designed to take automatic control of the frictional units, gear ratio selection and gear shifting. In general, the major components featured in such an automatic transmission are: a torque converter as above-mentioned; fluid pressure-operated multi-plate drive or brake clutches and/or brake bands which are connected to the individual elements of the gearsets in order to perform gear shifts without interrupting the tractive power; one-way clutches in conjunction with the frictional units for optimization of power shifts; and transmission controls, such as valves, for applying and releasing elements to shift the gears (instant of shifting), for enabling power shifting, and for choosing the proper gear (shift point control), dependent on shift-program selection by the driver (selector lever), accelerator position, the engine condition and vehicle speed.
The control system of the automatic transmission is typically hydraulically operated through the use of several valves to direct and regulate the supply of pressure. This hydraulic pressure control causes either the actuation or deactuation of the respective frictional units for effecting gear changes in the transmission. The valves used in the hydraulic control circuit typically comprise spring-biased spool valves, spring-biased accumulators and ball check valves. Since many of these valves rely upon springs to provide a predetermined amount of force, it should be appreciated that each transmission design represents a finely tuned arrangement of interdependent valve components. Although this type of transmission control system has worked adequately over the years, it does have its limitations. While each transmission is designed to operate most efficiently within certain specific tolerances, hydraulic control systems are typically incapable of taking self-corrective action to maintain operation of the transmission at peak efficiency. In particular, such hydraulically controlled transmission systems cannot readily adjust themselves in the field to compensate for varying environmental conditions that often affect the operational efficiency of the vehicle transmission.
A particular problem with current automatic transmission designs is control of an oncoming clutch when the vehicle is operated at high altitudes. In order to provide a smooth shift of the transmission, once the oncoming clutch has sufficient capacity, a hydraulic pressure request input to that clutch is normally changed to an upward slope until clutch engagement is complete. However, due to a decrease in engine torque of normally-aspirated engines at high altitudes, a pressure request that would be appropriate at sea level is often too high when the vehicle is operated at higher altitudes. This can result in an aggressive and sudden engagement of the oncoming clutch, which in turn can cause an aggressive, abrupt shift of the transmission. There is a need for a way to adjust the operation of a transmission clutch to compensate for variations in altitude, so that the oncoming clutch is actuated appropriately for the engine torque available at the altitude at which the vehicle is operated.